Loudspeaker system

ABSTRACT

A loudspeaker system which achieves a relatively uniform increase in response over a particular low-frequency range by means of a preselected center-to-center spacing of from about 6 inches to about 9 inches between two direct-radiating bass speakers which are mounted in substantially the same plane on the face baffle of a sealed enclosure and which are connected in parallel with the same polarity. Such a dual woofer speaker system has an improved frequency response rolloff characteristics and provides improved reproduction of the impact sound of percussion instruments.

United States Patent Long [54] LOUDSPEAKER SYSTEM [72] Inventor: Edward M. Long, Elk Grove Village, Ill.

[73] Assignee: Ampex Corporation, Redwood City, Calif.

[22] Filed: Jan. 26, 1970 [21] Appl. No.: 5,787

[52] US. Cl. ..l8l/31 B, 179/1 E [51] Int. Cl. ..G10k 13/00, H04r 1/28 [58] FieldofSearch ..l81/3l R,3l B, 27; 179/1 [56] References Cited UNITED STATES PATENTS 2,602,860 7/1952 Doubt ..l79/1 3,022,377 2/1962 Bobb et a1 3,241,631 3/1966 Manieri ..181/31 B FOREIGN PATENTS OR APPLICATIONS 165,502 3/1950 Austria ..l8l/3l B 5] Feb. 29,1972

Primary ExaminerStephen J. Tomsky Att0rney-Anderson, Luedeka, Fitch, Even & Tabin and RobertG.Clay

[57] ABSTRACT A loudspeaker system which achieves a relatively uniform increase in response over a particular low-frequency range by means of a preselected center-to-center spacing of from about 6 inches to about 9 inches between two direct-radiating bass speakers which are mounted in substantially the same plane on the face baffle of a sealed enclosure and which are connected in parallel with the same polarity. Such a dual woofer speaker system has an improved frequency response rolloff characteristics and provides improved reproduction of the impact sound of percussion instruments.

7 Claims, 5 Drawing Figures PATENftnrzazs 1972 E El Pica m HEKTZ \NVE'NTOE. EDWARD M Lo/vs ATT'YG.

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LOUDSPEAKER SYSTEM This invention generally relates to loudspeaker systems having an improved system performance. More particularly, the invention relates to a loudspeaker system having an improved response in a particular region of its bass frequency range.

It is desirable for the accurate reproduction of complex sounds having a wide variety of frequency components, such as music, that the output frequency response from loudspeaker systems be relatively uniform throughout the audiofrequency range.

Conventionally, separate speakers having maximized performance over different frequency ranges are combined in an attempt to provide uniform response over the audiofrequency range. For example, the lower frequency range output of a conventional loudspeaker system may be provided by a speaker designed for optimum bass performance, know as a woofer," and the upper frequency range output may be provided by a second speaker designed for optimum highfrequency performance, known as a tweeter. Midrange speakers may also be added to the system. Many such multispeaker systems, such as some small bookshelf-type systems in which the speakers are mounted on a flat bafile in the face of a small, sealed enclosure, seem unable, in practical use, to realistically reproduce impact sounds such as those from percussion instruments. This seems to be due primarily to a lack of proper balance in the acoustic output.

If the acoustic output in the range from about 200 to 600 Hz. is raised slightly with respect to the acoustic output at the bass resonance frequency of the loudspeaker system, these impact sounds are more realistically reproduced.

While it is possible to compensate for such performance deficiencies of a particular loudspeaker system through means such as appropriate compensatory design of the amplifier system to be associated with it, such an approach has the disadvantages of added amplifier complexity, as well as lack of component flexibility and interchangeability. A loudspeaker system having relatively flat response over its bandwidth would have the advantage of being useful with any modern high damping factor amplifier.

The design of the individual speakers for use in such multispeaker systems involves consideration and balancing of many complex and sometimes conflicting factors such as sensitivity, transient response, distortion, efficiency, and economy. Individual speakers are usually designed and constructed in an attempt to provide relatively flat response over a particular bandwidth with the response decreasing from the frequency limits of the intended useful bandwidth, (hereinafter referred to as roll off regions of the speaker). By providing an increased rate of rolloff below the lower frequency limit by acoustical means, undesirable speaker cone excursions are limited, thereby permitting improvement in speaker performance and design. Moreover, increased rolloff above the upper frequency limit facilitates the blending of response of the individual speakers in multispeaker systems.

An object of the present invention is the provision of a dual v woofer loudspeaker system having an improved balance between the low-frequency response at the system resonance and the response in the range from about 200 to 600 Hz. when compared to a loudspeaker system utilizing a single woofer of equivalent total piston area. It is another object to provide a speaker system having an increased output over a particular frequency range between approximately 200 and 600 Hz. It is a further object to provide a loudspeaker system which has improved ability to reproduce impact sounds such as those of percussion instruments. An additional object is the provision of a speaker system with improved low-frequency loudspeaker rolloff characteristics. 1

These and other objects of the invention are more particularly set forth in the following detailed description and in the accompanying drawings of which:

FIG. 1 is a schematic horizontal cross section of a speaker system in accordance with the invention;

FIG. 2 is a front view of the speaker system of FIG. I to which has been added a high-frequency speaker and a suitable crossover network;

FIG. 3 is a graphical representation showing by a dotted line the acoustical response of one of the low-frequency speakers shown in FIG. 1 when mounted in an enclosure of a volume one-half that of the enclosure of FIGS. 1 and 2 and by a solid line the acoustic response of the two lowfrequency loudspeakers of FIG. 1;

FIG. 4 shows the graphical representations of FIG. 3 in normalized relationship to illustrate the advantages of this invention; and

FIG. 5 is three graphical representations, each of which shows the acoustical power output for two loudspeakers of FIG. 1 as compared to that of a single loudspeaker with the same electrical input power, the zero reference line representing the output of the single loudspeaker.

With reference to FIG. 1 and FIG. 2 of the drawings, a loudspeaker system is shown which achieves a response enhancement over a particular frequency range by means of the acoustic interaction between two direct-radiating lowfrequency speakers 11. The loudspeakers 11 are mounted in a substantially coplanar manner on a speaker bafile l2 forming the face of a sealed enclosure 14. The loudspeakers 11 are mounted on the baffle 12 with a predetermined center-tocenter spacing, indicated by the letter A in FIG. I, and are connected in parallel, and with the same polarity to the driving signal. The response enhancement provided by this invention is produced through the mutual acoustic impedance coupling of the loudspeakers 11 over a particular frequency range in the bandwidth, the range of enhancement being a function of the center-to-center spacing of the loudspeakers.

More particularly, the loudspeaker system shown in the drawings includes two direct radiating, loudspeakers I], which are preferably identical in construction. The loudspeakers have a nominal diameter from about 5% to about 8 inches. To obtain some of the benefits of the invention, such loudspeakers should have a frequency response bandwidth which includes at least the frequency range of from about 200 to about 600 Hz. Preferably, to obtain maximum advantage of the invention, the loudspeakers should have a bandwidth which includes at least the range of from about I00 to about 2,000 Hz. The illustrated direct radiating loudspeakers are moving coil or electrodynamic speakers in which the audiofrequency signal is passed through a voice coil mounted in a magnetic field and loaded by the speaker cone. However, reed armature and balanced armature electromagnetic speakers which employ a spring to return the armature to its equilibrium position may be employed. Also, inductor dynamic speakers which have two balanced annatures and do not require mechanical springs may be employed.

In the embodiment illustrated in FIGS. 1 and 2, the bass end speakers 11 are mounted on the bafile 12, which in the illustrated embodiment is flat, in a substantially coplanar manner. The bafile 12 forms the face of the sealed enclosure 14 so that acoustic radiation from the rear surfaces of the speaker cones is substantially isolated therein, and thereby prevents substantial interaction of the same with acoustic radiation from the front cone surfaces of the loudspeakers 11. Thus, most of the external radiation originates from the front cone surfaces of the loudspeakers 11.

To enhance a particular frequency range in the bandwidth of the speakers, the loudspeakers are mounted so that they have a predetermined center-to-center spacing (the dimension A in FIG. 1) of between about 6 and about 9 inches. This range of positioning has been found to provide a combination speakers may somewhat restrict the minimum limit of the range of spacing positions which may be used for those speakers. For example, two loudspeakers each having a nominal diameter of 8 inches may be mounted in an essentially coplanar manner with center-to-center spacings having a minimum of approximately 8 inches.

The frequency range of acoustic enhancement is generally inversely related, with respect to both its frequency range and its center frequency, to the centerto-center spacing of the bass transducers. Thus, the enhanced range is narrower and the center frequency is lower for greater center-to-center spacings. Conversely, enhanced range is wider and the center frequency is higher for smaller center-to-center spacings. The curves of FIG. 5 show the gain in acoustical power for three different size dual woofers (viz. 5%, 6, and 8 inches) over that of a single loudspeaker of the same respective size, with the same electrical input power and mounted in an enclosure of a volume one-half that of the enclosure of the dual speakers. The zero reference line represents the output of the single loudspeaker, and the size and spacing the loudspeaker is indicated in FIG. 5. The curves of FIG. 5 represent the total power output from the loudspeaker system. They are obtained by placing the respective system in a reverberation chamber and measuring the total integrated acoustical power output.

With 5% inch' bass speakers and a center-to-center spacing of about 6 inches, the frequency of maximum gain over a single 5% inch bass speaker is about 550 Hz. and the range of response enhancement extends between about 200 and about 1,500 Hz. With 8-inch bass speakers and a center-to-center spacing of about 9 inches, the frequency of maximum gain over a single 8-inch bass speaker is about 370 Hz. and the range of response enhancement extends between about 150 and about 800 Hz. Intermediate center-to-center spacings provide a proportionately intermediate range of response enhancement, but the optimum spacing is a value only slightly over the minimum allowable for each size. This would mean spacing of about 6 inches for a 5% inch size, about 7% inches for a 6 inch size and about 9 inches for an 8 inch size. For two identical loudspeakers positioned as described herein, for the same power input, the total acoustic radiation is increased substantially throughout the entire bandwidth of the loudspeakers and throughout most of the lower rolloff region, as compared to the acoustic radiation of one loudspeaker. The maximum increase in acoustic radiation is slightly more than twice that of one such loudspeaker. In the range of maximum response enhancement, the increase in response as compared to the increase in response in other regions is about 3 decibels. In addition, at frequencies somewhat below the low-frequency system resonance of the two coupled loudspeakers, the response tends to be increasingly reduced with decreasing frequency in comparison to the response of one loudspeaker. The acoustic interaction of the properly positioned and mounted loudspeakers provides an increased acoustical reactance at frequencies lower than the particular frequency range. An increased rate of rolloff below the lower frequency limit is thus provided which reduces cone excursions at those lower frequencies for the same power input. The ability to acoustically reduce the maximum required cone excursion pennits improved speaker cone, cone support, magnet, and voice coil design.

In order thatthe full audio bandwidth may be reproduced, it is also contemplated that the two loudspeakers 11 will be employed with at least one additional speaker 15 designed for highfrequency performance, and an appropriately designed crossover network 16, to achieve relatively flat response blending. Midrange speakers may also be used. Response blending is facilitated by the increased rate of rolloff above the upper frequency limit of the two properly positioned and mounted loudspeakers 11. The loudspeakers are connected in parallel with the driving signal, and additionally are connected withthe same polarity so that they are driven in phase.

To illustrate the advantages of the present invention, a comparison is made hereinafter between the response of a single bass loudspeaker and the response of two bass loudspeakers constructed in accordance with the present invention. The single speaker is a dynamic, moving coil loudspeaker having a nominal diameter of 6 inches and an impedance of 8 ohms, which has a relatively flat frequency response from about 3,000 Hz. down to the bass resonance frequency of about 70 Hz. The single loudspeaker is mounted on a flat baffle in the face of a small sealed enclosure having a volume of 650 cubic inches. The loudspeaker is then supplied with a uniform input over the frequency range from 20 to 20 kHz. The acoustic output from this loudspeaker is measured at a point 18 inches directly in front of the speaker disposed in an anechoic chamber. The response curve is shown by the dotted line in FIG. 3.

Two 6inch bass loudspeakers which are identical to this single bass loudspeaker, are mounted on a flat baffle in the face of a sealed enclosure having a volume of L365 cubic inches, so that they have a center-to-center spacing of 7.25 inches. The two loudspeakers are connected in parallel and with the same polarity to the audio input supplied above to the single mounted loudspeaker. The acoustical response from this dual woofer loudspeaker system is measured from a point 18 inches directly in front of and between the two bass loudspeakers. The frequency response curve is shown by the solid line in FIG. 3. In FIG. 4, the curves of FIG. 3 are drawn in normalized relationship so that the response enhancement of from about 200 to about 600 Hz., may be more clearly seen. In addition, FIG. 4 shows the improved rolloff of the dual woofer loudspeaker system, resulting in reduced maximum cone excursions below the system resonance. A tweeter designed for $2.5 db. response over the frequency range from 700 to 17 kHz. is then mounted on the flat bafile. The tweeter and dual woofers are connected to the audio signal by means of a crossover network with an acoustical crossover frequency of 1,400 Hz., with the tweeter reproducing only the high-frequency band and the two bass loudspeakers reproducing the lowfrequency band.

Various features of the invention are set forth in the following claims.

What is claimed is:

1. A sealed enclosure loudspeaker system with improved capability for reproducing impact sounds, and having an acoustically provided increase in acoustic output over a particular region of its bass frequency range including the range of from about 200 to about 600 Hz. and an acoustically provided improvement in rolloff characteristics, comprising, in combination, a sealed speaker enclosure having a speaker baffle fonning its face, and two loudspeakers mounted on said baffle such that acoustic radiation from the rear surfaces of the speaker cones of said loudspeakers is substantially isolated in said speaker enclosure to prevent substantial interaction of such rear speaker cone radiation with the acoustic radiation from the front cone surfaces of said loudspeakers, said loudspeakers being mounted in a substantially coplanar manner on said baffle with a preselected center-to-center spacing of between about 6 and about 9 inches, said loudspeakers having nominal diameters of between about 5 /4 and about 8 inches and a bandwidth which includes at least the range of from about to about 2,000 Hz., and being connected in parallel and with the same polarity to the driving signal, such that an enhancement of response over said particular region of the bass frequency range is produced through the mutual acoustic impedance coupling of said loudspeakers andsuch that an increased rate of rolloff is provided below the lower frequency limit and above the upper frequency limit of the bandwidth of said loudspeakers. 1

2. A loudspeaker system according to claim 1 wherein said loudspeakers are mounted with a preselected center-to-center spacing slightly larger than the minimum spacing allowed by the diameter of the loudspeakers.

3. A loudspeaker system according to claim 1 wherein said loudspeakers have nominal diameters of about 6 inches, and wherein said loudspeakers are mounted with a preselected center-to-center spacing of about 7 /4 inches.

4. A loudspeaker system according to claim I wherein said loudspeakers have nominal diameters of about 5% inches, and wherein said loudspeakers are mounted with a preselected center-to-center spacing of about 6 inches.

5. A loudspeaker system according to claim 1 wherein said loudspeakers have nominal diameters of about 8 inches, and wherein said loudspeakers are mounted with a preselected center-to-centcr spacing of about 9 inches.

6. A loudspeaker system according to claim 2 wherein said loudspeaker system includes a high-frequency range speaker in addition to said two loudspeakers and wherein said loudspeakers are connected to the driving signal through a crossover network which supplies the lower frequency components of the driving signal to said loudspeakers and the higher frequency components of the driving signal to said 7 

1. A sealed enclosure loudspeaker system with improved capability for reproducing impact sounds, and having an acoustically provided increase in acoustic output over a particular region of its bass frequency range including the range of from about 200 to about 600 Hz. and an acoustically provided improvement in rolloff characteristics, comprising, in combination, a sealed speaker enclosure having a speaker baffle forming its face, and two loudspeakers mounted on said baffle such that acoustic radiation from the rear surfaces of the speaker cones of said loudspeakers is substantially isolated in said speaker enclosure to prevent substantial interaction of such rear speaker cone radiation with the acoustic radiation from the front cone surfaces of said loudspeakers, said loudspeakers being mounted in a substantially coplanar manner on said baffle with a preselected center-to-center spacing of between about 6 and about 9 inches, said loudspeakers having nominal diameters of between about 5 1/4 and about 8 inches and a bandwidth which includes at least the range of from about 100 to about 2,000 Hz., and being connected in parallel and with the same polarity to the driving signal, such that an enhancement of response over said particular region of the bass frequency range is produced through the mutual acoustic impedance coupling of said loudspeakers and such that an increased rate of rolloff is provided below the lower frequency limit and above the upper frequency limit of the bandwidth of said loudspeakers.
 2. A loudspeaker system according to claim 1 wherein said loudspeakers are mounted with a preselected center-to-center spacing slightly larger than the minimum spacing allowed by the diameter of the loudspeakers.
 3. A loudspeaker system according to claim 1 wherein said loudspeakers have nominal diameters of about 6 inches, and wherein said loudspeakers are mounted with a preselected center-to-center spacing of about 7 1/4 inches.
 4. A loudspeaker system according to claim 1 wherein said loudspeakers have nominal diameters of about 5 1/4 inches, and wherein said loudspeakers are mounted with a preselected center-to-center spacing of about 6 inches.
 5. A loudspeaker system according to claim 1 wherein said loudspeakers have nominal diameters of about 8 inches, and wherein said loudspeakers are mounted with a preselected center-to-center spacing of about 9 inches.
 6. A loudspeaker system according to claim 2 wherein said loudspeaker system includes a high-frequency range speaker in addition to said two loudspeakers and wherein said loudspeakers are connected to the driving signal through a crossover network which supplies the lower frequency components of the driving signal to said loudspeakers and the higher frequency components of the driving signal to said high-frequency range speaker.
 7. A loudspeaker system according to claim 6 wherein said two loudspeakers are identical electrodynamic moving coil loudspeakers having a relatively flat frequency response from about 3,000 Hz. down to their bass resonance frequency of about 70 Hz. wherein said sealed enclosure has a volume of about 1,365 cubic inches, and wherein said preselected center-to-center spacing of said speakers is about 7 1/4 inches. 